1. Field of the Invention
The present invention relates to a burner for manufacturing an optical fiber preform used to make optical glass fibers and to a method for producing said optical, preforms.
In particular, the present invention relates to a multi-flame deposition burner suitable for manufacturing optical preforms in an efficient and stable manner at high fabrication rate and to a method for producing an optical preform by using said burner.
2. Background Art
Glass fibers for optical communication are made from high purity, silica-based glass fibers drawn from glass preforms, which preforms are produced according to various glass deposition techniques.
Some of these deposition techniques, including vapor axial deposition (VAD) and outside vapor deposition (OVD), are based on flame combustion wherein reactants (i.e. silica precursors, such as SiCl4, optionally together with dopants materials, such as GeCl4, for suitably modifying the refractive index of the glass) are fed together with combusting gases through a deposition burner which directs a high temperature flow of forming fine glass particles onto a rotating growing target preform.
According to the VAD deposition technique, the growth of the preform takes place in an axial direction. Thus, the deposition burner(s) is typically maintained in a substantially fixed position, while the rotating preform is slowly moved upwardly (or downwardly) with respect to the burner, in order to cause the axial growth of the preform. Alternatively, the rotating preform can be maintained in a substantially fixed position, while the deposition burner is slowly moved downwardly (or upwardly) with respect to the preform.
Differently from the VAD technique, in the OVD technique the growth of the preform takes place in a radial direction. In this case, a rotating target (e.g. a quartz glass rod) is generally positioned in a fixed horizontal or vertical position and the deposition burner is repeatedly passed along the surface of the growing preform for causing the radial growth of the same.
Independently from the applied deposition technique, a porous glass preform is thus fabricated, which is then consolidated to form a solid glass preform apt for being subsequently drawn into an optical fiber.
Typically, an optical preform comprises a central portion (core) and an outer portion (cladding), the core and the cladding differing in their respective chemical composition and having thus different refractive indexes. As in the optical fibers, the cladding portion forms the majority of the preform. The preform is typically manufactured by producing and consolidating a first preform comprising the core and a first portion of the cladding. An overcladding layer is then deposited onto said first preform, thus obtaining a porous preform, which is then consolidated into the final preform.
In general, conventional burners for manufacturing optical preforms are made up of a plurality of co-axial pipes through which the glass precursor materials (i.e. silica precursors, such as SiCl4, optionally together with dopants materials, such as GeCl4), the combusting gases (e.g. oxygen and hydrogen or methane) and, optionally, some inert gas (e.g. argon or helium) are fed. Typically, the glass precursor material is fed through the central pipe of the burner, while other gases are fed through the annular openings formed by the concentrically disposed pipes.
Examples of such conventional burners are disclosed, for instance, in U.S. Pat. Nos. 4,345,928, 4,465,708, 4,474,593, 4,661,140, and 4,810,189.
“Multi-flame” burners, generating a plurality of independent flames disposed concentrically one to each other, are also disclosed. For instance, U.S. Pat. No. 4,801,322 discloses a multi-flame burner wherein the inner flame, including a glass precursor material, is positioned rearwardly of the outer flame. As mentioned in said patent, the outer flame allows to increase the flame length with consequent size increase of the synthesised glass particles.
U.S. Pat. No. 4,826,520 discloses a modified multi-flame burner for producing doped optical preforms wherein a central pipe, through which a doping reactant (GeCl4) is fed, is spaced forwardly with respect to the other pipes forming the inner flame, in order to reduce the staying time of the doping material inside the flame.
Although few prior art documents disclose burner having pipes made from a heat resistant metallic cylindrical material (e.g. U.S. Pat. No. 4,661,140), the pipes of conventional prior art burners are generally and more desirably made from quartz glass or ceramic materials, as disclosed for instance in U.S. Pat. No. 4,345,928 (col. 8, lines 52-55), U.S. Pat. No. 4,474,593 (col. 2, lines 16-19), U.S. Pat. No. 4,465,708 (col. 1, lines 58-61), U.S. Pat. No. 4,801,322 (col. 26, lines 32-40) and U.S. Pat. No. 4,810,189 (col. 4, lines 66-68).
As a matter of fact, quartz or ceramic materials are more heat resistant than metallic materials to high temperatures and may thus more easily withstand the typical temperature developed by the flame in the burner. In any case, the possible use of heat resistant metallic pipes in conventional deposition burner is necessarily limited to the single-flame type burners (such as the one disclosed in U.S. Pat. No. 4,661,140). In these burners, all the co-axial pipes through which reactants/inert gases flow have in fact substantially the same length; the overheating of said pipes is thus avoided by maintaining the flame sufficiently spaced apart from the tips of the pipes.
However, as observed by the Applicant, in the multi-flame burners of the prior art, such as the one disclosed in U.S. Pat. No. 4,801,322, problems may arise in using metallic materials for manufacturing the pipes of the burner. In fact, as disclosed in the above cited patent, the pipes generating outer flame are longer than the pipes generating the inner flame, in order to obtain the rearward spacing of the inner flame with respect to the outer flame. Thus, the inner surface of the pipes forming the outer flame is subjected to the heat generated by the inner flame. While the typical temperature of a flame is of about 2500-3000° C., the surface of the pipes exposed to the flame may reach a temperature of several hundreds degrees, typically of about 600-800° C. As it is apparent that a pipe made from a metallic material can not withstand the heat generated by such a flame, it is therefore necessary, as mentioned in the above cited patent, to use a burner with quartz glass pipes. This problem is much more evident for burners specifically designed for the outer cladding deposition, which produce larger flames and accordingly higher amount of heat.
The Applicant has however observed that the use of quartz glass or ceramic materials for producing a burner results in a number of drawbacks. In particular, the concentricity of glass pipes is rather cumbersome to obtain and particular attention shall be paid to the relative alignment of the co-axial pipes. In addition, a burner containing a plurality of glass pipes shall be handled with care for avoiding possible damages of the pipes.
The Applicant has now found that in a multi-flame burner, comprising at least an inner section comprising a first plurality of ducts for generating an inner flame and at least an outer section comprising a second plurality of ducts for generating an outer flame surrounding said inner flame, said inner flame can be advantageously confined and separated from the outer flame by disposing a separating tube made of a heat resistant material, in particular of quartz glass or ceramic material (e.g. alumina), between said inner and said outer section.
According to such a burner design, the pipes forming the ducts of the multi-flame burner may thus advantageously be made from a metallic material, e.g. stainless steel.
In addition, as observed by the Applicant, while the burners for depositing the core and the inner cladding of the preform are generally of reduced dimensions, the burner used for depositing the overcladding, in particular for large dimensions preforms, shall be relatively larger, in order to allow the generation of higher flow rates which are necessary for increasing the amount of deposited material, maintaining at the same time the velocity of the gases relatively low.
The Applicant has thus observed that, particularly for overcladding deposition and especially when manufacturing large diameter optical preforms, the deposition rate of the process can be increased by suitably modifying the geometry of deposition burner in order to redistribute the flow of fine glass particles impacting onto the target preform. In particular, it has been observed that the shape of said flow can be advantageously modified in its terminal portion before impacting onto the target preform, by increasing the dimension of said flow in a direction substantially perpendicular to the longitudinal axis of said target preform.
The modification of the geometry of the flow of glass particles is particularly easy and effective when applied onto a multi-flame burner with a. single flame-separating tube according to the present invention.